Natural gas generally refers to rarefied or gaseous hydrocarbons (methane and higher hydrocarbons such as ethane, propane, butane, and the like) which are found in the earth. Non-combustible gases occurring in the earth, such as carbon dioxide, helium and nitrogen are generally referred to by their proper chemical names. Often, however, non-combustible gases are found in combination with combustible gases and the mixture is referred to generally as “natural gas” without any attempt to distinguish between combustible and non-combustible gases. See Pruitt, “Mineral Terms-Some Problems in Their Use and Definition,” Rocky Mt. Min. L. Rev. 1, 16 (1966).
Natural gas is often plentiful in regions where it is uneconomical to develop the reserves due to lack of a local market for the gas or the high cost of processing and transporting the gas to distant markets.
It is common practice to cryogenically liquefy natural gas so as to produce liquefied natural gas (LNG) for storage and transport. A fundamental reason for the liquefaction of natural gas is that liquefaction results in a volume reduction of about 1/600, thereby making it possible to store and transport the liquefied gas in containers at low or even atmospheric pressure. Liquefaction of natural gas is of even greater importance in enabling the transport of gas from a supply source to market where the source and market are separated by great distances and pipeline transport is not practical or economically feasible.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to −240° F. (−151° C.) to −260° F. (−162° C.) where it may exist as a liquid at near atmospheric pressure. Various methods and/or systems exist in the prior art for liquefying natural gas or the like whereby the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages, and cooling the gas to successively lower temperatures until liquefaction is achieved. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, nitrogen and methane, or mixtures thereof. The refrigerants are commonly arranged in a cascaded manner, in order of diminishing refrigerant boiling point. For example, processes for preparation of LNG generally are disclosed in U.S. Pat. Nos. 4,445,917; 5,537,827; 6,023,942; 6,041,619; 6,062,041; 6,248,794, and UK Patent Application GB 2,357,140 A. The teachings of these patents are incorporated herein by reference in their entirety.
Additionally, chilled, pressurized natural gas can be expanded to atmospheric pressure by passing the natural gas through one or more expansion stages. During the course of this expansion to atmospheric pressure, the gas is further cooled to a suitable storage or transport temperature by flash vaporizing at least a portion of the already liquefied natural gas. The flashed vapors from the expansion stages are generally collected and recycled for liquefaction or burned to generate power for the LNG manufacturing facility.
LNG projects have not always been economical in that cryogenic refrigeration systems are highly energy intensive and require a substantial capital investment. In addition, participating in the LNG business requires further investment for sophisticated and costly shipping vessels and regasification systems so that the LNG consumer can process the product.
An alternative to the cryogenic liquefaction of natural gas to LNG is the chemical conversion of natural gas into GTL (GTL) products. Methods for producing GTL products can be conveniently categorized as indirect synthesis gas routes or as direct routes. The indirect synthesis gas routes involve the production of synthesis gas comprising hydrogen and carbon dioxide as an intermediate product whereas, for purposes of the present invention, the direct routes shall be construed as covering all others.
Traditional GTL products include, but are not limited to, hydrogen, methanol, acetic acid, olefins, dimethyl ether, dimethoxy methane, polydimethoxy methane, urea, ammonia, fertilizer and Fischer Tropsch reaction products. The Fischer Tropsch reaction produces mostly paraffinic products of varying carbon chain length, useful for producing lower boiling alkanes, naphtha, distillates useful as jet and diesel fuel and furnace oil, and lubricating oil and wax base stocks.
The most common commercial methods for producing synthesis gas are steam-methane reforming, auto-thermal reforming, gas heated reforming, partial oxidation, and combinations thereof.                Steam methane reforming generally reacts steam and natural gas at high temperatures and moderate pressures over a reduced nickel-containing catalyst to produce synthesis gas.        Autothermal reforming generally processes steam, natural gas and oxygen through a specialized burner where only a portion of the methane from the natural gas is combusted. Partial combustion of the natural gas provides the heat necessary to conduct the reforming reactions that will occur over a catalyst bed located in proximity to the burner.        Gas heated reforming consists of two reactors or reaction zones, a gas heated reformer reactor/zone and an autothermal reformer reactor/zone. Steam and natural gas are fed to the gas-heated reformer where a portion of the natural gas reacts, over catalyst, to form synthesis gas. This mixture of unreacted natural gas and synthesis gas is then fed to the autothermal reformer, along with oxygen, where the remaining natural gas is converted to synthesis gas. The hot synthesis gas stream exiting the autothermal reformer is then routed back to the gas reformer to provide the heat of reaction necessary for the gas-heated reformer.        Partial oxidation reforming generally processes steam, natural gas and oxygen through a specialized burner where a substantial portion of the methane is combusted at high temperatures to produce synthesis gas. Contrary to autothermal reforming, no catalyst is present in the partial oxidation reactor.        
Current technology for manufacturing synthesis gas is highly capital intensive. Autothermal and partial oxidative synthesis gas methods generally require a costly air separation plant to produce oxygen. Steam methane reforming on the other hand, does not require oxygen manufacture.
Natural gas reserve holders have found that substantially increasing the capacity of a LNG or GTL plant can improve plant construction economics. Many of the costs inherent to building such plants are fixed or minimally, do not increase linearly with capacity. However, it has also been found that as more of a single product is produced in a distinct and often isolated geographical region, the product price over cost margin for blocks of product if not all of the plant output is reduced.
Integrating a LNG plant and a GTL plant offers the potential for producing a portfolio of products which can turn projects that would not have been commercially viable for many of the above noted reasons into viable projects. While it is believed that there have been no integrated LNG/GTL plants built to date, there has been increased interest in combining both technologies at a single plant site.                For example, Geijsel et al., “Synergies Between LNG and GTL Conversion,” The 13th International Conference & Exhibition on Liquefied Natural Gas, Seoul, Korea, May 14–17, discloses potential benefits for combining a Fischer Tropsch GTL plant (utilizing a combined partial oxidation/steam reforming synthesis gas preparation step) with LNG manufacture.        
U.S. Pat. No. 6,248,794 to Gieskes similarly discloses a method for utilizing tail gas from a Fischer Tropsch GTL plant as fuel for a refrigeration plant at an LNG facility.
Commonly assigned co-pending U.S. patent application Ser. No. 10/051,425, filed Jan. 18, 2002, discloses a method for utilizing flash gas from an LNG process as feed for a GTL process making GTL products. The teachings of this application are incorporated by reference herein in their entirety.
The above-referenced teachings in the area of integrated LNG with GTL technology are largely directed to the sharing of common plant infrastructure and utilities and other incremental consolidation improvements.
U.K. Patent Application GB 2357140 to Rummelhoff is directed to a process for integrating natural gas liquids (NGL) recovery, LNG production and methanol manufacture. The Rummelhoff process performs two expansion and separation steps so as to provide energy recovery sufficient to facilitate the separation of higher boiling natural gas liquids (“NGLs”) such as ethane and higher boiling point hydrocarbon) from LNG. Subsequent to NGL recovery, the Rummelhoff process provides a single, final stage of expanding and separating so as to remove a natural gas stream from LNG for conveying to post-processing steps such as the production of methanol.
U.S. Pat. No. 6,180,684 to Halmo et al. discloses integrating the production of synthetic fuel and electrical power generation. While the process disclosed therein provides for separation of acid gases, such as CO2, from a feed stream directed to LNG production, the CO2 obtained thereby is subsequently directed to reforming processes which require oxygen to prepare synthesis gas.
At present, commercial scale LNG plants use processes which generally require nearly complete removal of acid gases, including CO2, from the feed gas. In the past, the CO2 extracted from the feed gas has been simply vented to the atmosphere. However, current concerns over global warming, internationally-driven initiatives to reduce greenhouse emissions, and other environmental factors make venting of such CO2 undesirable.